1. Field of the Invention
The present invention relates to a light emitting diode array, in particular, to an improvement of a monolithic light emitting diode array having a plurality of surface light emitting diodes having a mesa structure aligned in row, and to the production method of the light emitting diode array.
2. Description of the Related Art
Presently, in order to utilize light emitting diodes (referred to as LEDs hereafter) as a light source for a printer utilizing a light and for an information writing element of an optical information device, there are proposed or developed many techniques for integrating a plurality of LEDs into a monolithic LED array (referred to as LED array hereafter) in a high integration.
The LED array is provided with an array of LEDs having an integration of, for instance, 300-400 dpi (dots per inch) by causing the LEDs to be lineally aligned in row, however, the LED array is further required to have a higher integration because of demands for down-sizing of the light utilizing printer and for a high definition picture of the optical information device.
In order to realize an LED array having an integration of more than 1600 dpi, the LEDs have to be aligned in row at a pitch of less than 16 .mu.m on a substrate crystal. As a method for electrically isolating the LEDs from each other, there is proposed such a method as providing isolating mesa grooves between the LEDs by using an etching liquid or such a method as diffusing impurity elements to form diffusion areas between the LEDs.
In the above etching method and impurity diffusion method, the isolating mesa grooves and the diffusion areas are spread not only in directions of depths thereof but also in lateral directions thereof, so that the grooves and the diffusion areas have to be formed in the depths of less than 3 .mu.m. The technique for isolating the LEDs from each other by the impurity diffusion method is disclosed in Japanese Laid-Open Publication 2382/1984.
In the production method of the prior art, at first, a p-type GaAlAs layer and an n-type GaAlAs layer are formed on a substrate crystal in this order by a liquid phase growth method. Thus, a semi-conductive wafer is obtained. Next, after an insulation layer, for instance, of SiO.sub.2 is formed on the semi-conductor wafer, one part of the insulation layer is removed by using a photolithography leaving another part thereof corresponding to the light emitting areas. Then, zinc elements are diffused into the n-type GaAlAs layer so as to reach the p-type GaAlAs layer from the surface of the wafer by causing the insulation layer to be a mask, so that p-type isolation areas are formed in the semi-conductive wafer, thus the LED array having a plurality of separated LEDs is obtained. According to the above LED array, it is possible to obtain an LED array having an aligned pitch of less than 50 .mu.m by causing a thickness of the n-type GaAlAs on which the p-type isolation areas are formed, to be less than 10 .mu.m.
In the above production method of the LED array, however, it was difficult to obtain the LED array having a high light emitting output because of the impurity diffusion.
Further, it was difficult to obtain the LED array having an adequately small dispersion of light emitting output because a layer having thickness of less than 10 .mu.m can not be obtained by the liquid phase growth method without dispersion of the thickness thereof, while the liquid phase growth method is suitable to form a layer having a thickness of more than 10 .mu.m.
Thus, the liquid phase growth method is employed to obtain a semi-conductive wafer having thick films on the substrate crystal. The thick films allow the semi-conductive wafer to remove the substrate crystal by using an etching liquid., so that a conductive layer for an electrode can be directly formed on the thick films after removing the substrate from the semi-conductive wafer. This structure of the LED array is advantageous to preventing the lights emitted from the light emitting areas from being absorbed by the substrate crystal.
Recently, new thin film forming techniques such as an organic metal vapor phase growth method and a molecular beam epitaxial growth method have been developed for forming a semi-conductive layer having a thickness of 5.about.6 .mu.m on the substrate crystal, and these techniques have realized a thin film having an uniform thickness because of excellent controllability of the thin film.
These organic metal vapor phase growth method and molecular beam epitaxial growth method have the excellent controllability of the thin films formed on the substrate crystal, however, they are not suitable for forming the thin films having thicknesses of more than 10 .mu.m on the contrary. Therefore, the conductive layer for electrode of the LED array has to be provided on the surface of the substrate crystal on which the thin films are formed because the thin films formed by the above methods are too thin or mechanically too weak to remove the substrate crystal from the semi-conductive wafer leaving thin films by using an etching liquid. This poses a loss of light emitting output because lights emitted from the diodes are mostly absorbed by the substrate crystal.
Further, in the prior art, in order to form the isolating mesa grooves between the emitting diodes, a mesa etching method using the etching liquid is employed.
When an isolating mesa groove is formed in a direction of a &lt;0 -1 -1&gt; axis of a crystal semi-conductor on a (1 0 0) plane by a mesa etching using HCl liquid, edge angles formed at side walls of the isolating mesa groove have acute angles or negative slopes (the groove having walls of the negative slopes is referred to as a negative-type mesa groove hereafter). On the other hand, when an isolating mesa groove is formed in a direction of a &lt;0 -1 1&gt; axis of the crystal on the (1 0 0) plane by the mesa etching, edge angles formed at side walls of the isolating mesa groove have obtuse angles or positive slopes (the groove having walls of positive slopes is referred to as a positive-type mesa groove hereafter).
Thus, when a direction of the LEDs aligned in row, i.e. a direction of an LED array, is defined in parallel with the &lt;0 -1 1&gt; axis of the crystal, the isolating mesa groove formed in parallel with the direction of the LED array becomes the positive-type mesa groove. On the other hand, a crystal axis vertical to the &lt;0 -1 1&gt; axis becomes a &lt;0 -1 -1&gt;, thus, the isolating mesa grooves for dividing an LED array portion to an individual LED become the negative-type mesa grooves.
As mentioned in the above, the isolating mesa grooves of the LEDs array have the negative-type grooves, thus, the deeper the depths of the negative-type grooves, the smaller the areas at the feet of the LEDs through which a current follows, so that each of the LEDs has a large resistance, which poses a cause of decrease of light emitting output.
Generally, upon a mass-production of the LED arrays, an electrode is formed on a top surface of each of the LEDs by forming an electrode pattern on a wafer made of semi-conductive layer using plasma etching. At first, an electrode layer is provided on the wafer having the isolating mesa grooves provided in advance by evaporating electrode material, then, the electrode layer except for parts provided on the top of LEDs is removed from the wafer by the plasma etching using a mask pattern. At that time, the electrode layer formed on the slopes and parts under the slopes in the negative-type grooves is apt to be left thereon because of shade of the LED portion, which poses a cause of short circuit. b